<HTML><HEAD><TITLE>X ~= Y</TITLE>
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<H1>X ~= Y</H1>
Constrains X and Y to be different
<H2>Description</H2>
Fails if X and Y are non-unifiable, otherwise succeeds
    or delays.  Unlike the implementation of the same predicate in the
    kernel, this one maintains and explicit wavefront and has only one
    delayed goal.  Failure is detected eagerly.  Success may be
    detected late.

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